At the present time it is known that faint electrical and magnetic waves in the human brain may be detected and analyzed by non-invasive methods. There are many patents and publications in the fields of EEG (electrocencephalogram) and MEG (magnetoencephalogram) which describe the medical benefits made possible by brain wave analysis. For example, it may be possible to detect neurological disorders such as epilepsy, and psychiatric disorders, such as unipolar depression.
The EEG is widely used in clinical practice and is commonly found in neurological clinics and hospitals. Although there has been considerable research activity and resulting publications using MEG, it is generally believed, especially by clinical neurologists, that better medical diagnoses may be obtained using EEG. However, it is usually necessary, using presently available EEG methods, to retain a subject in the EEG apparatus for a prolonged time period, over 30 minutes, to obtain a meaningful set of data. Appreciable additional time is required for electrode application and removal. However, MEG has not been widely used in a clinical setting, although it possesses certain advantages compared to EEG. These advantages include:
(i) In MEG the dewar container may be easily placed on or close to the patient's head. The pick-up detectors are within the dewar container and there may be as many as 122 or more magnetic-detectors able to detect magnetic brain wave effects at 122, or more, separate areas on the surface of the patient's scalp. In contrast, in EEG, it may be time-consuming, messy and even painful to make a satisfactory low impedance connection with the patient's scalp, especially if the patient has a large head of hair. Generally fewer electrodes are used, for example, a widely used electrode placement system is the "International 10/20 System" which provides 19 active EEG electrodes.
(ii) The time required for a testing session may be greater in the case of EEG than MEG. In EEG it is important to repeatedly test the impedance of the EEG electrode connection to the scalp. Often the patient moves and disturbs that connection, which requires that the electrode be re-connected. Such monitoring and re-connection is not required with MEG systems. In addition, it is believed that MEG is less adversely affected by tissue artifact, such as eye movement, than EEG.
(iii) The MEG has certain theoretical advantages over the EEG. In MEG the neuromagnetic field vector B provides directional information about the orientation of the source. The neuromagnetic field vector B is not distorted by passage through the brain and scalp, as are the EEG electrical brain wave signals. The MEG is an absolute measure of source strength and not measured with respect to a reference, i.e., the patient's body, as in EEG.
Despite these advantages, MEG has generally been used in universities or medical centers for research and not for clinical applications. One reason is cost, as MEG systems cost two to three million dollars. Another reason is the inability to analyze the MEG data to produce meaningful clinical results.
An MEG system may require a magnetic shielded room (high Mu room) which is large enough for the dewar part of the MEG system and the patient. The MEG system is sufficiently sensitive, if not enclosed in such a room, that the magnetic influence of even a passing car would create sufficient noise to drown out brain wave magnetic signals.
A typical MEG uses 50-130 magnetic detecting coils in a dewar (insulated vacuum container). Each of these coils is a Superconducting Quantum Interference Device (SQUID) which is operated at a cryogenic temperature. The coils may be brought as close to the subject's scalp as the thickness of the dewar, for example, 1 cm. The SQUID has one or two Josephson junctions which exhibit quantum interference effects. The MEG systems are generally large, complex and costly compared to EEG systems.
In one type of MEG system, available from Neuromag Oy, Finland, the patient sits in a chair and a dewar is lowered over the subject's head.
The dewar has a helmet-shaped cavity at its end ("dewar tail") which fits over the subject's head, and contains over 100 SQUID detectors, each of which becomes perpendicular to the subject's scalp.
Another system uses a clam-shell like arrangement in which the subject lays down with his head in a cavity in a bed-like setting and an upper shield having a head cavity is lowered over the subject. That system does not have SQUID detectors at the mid-line of the scalp. In addition, the subject is lying down and not in a good position to respond to stimuli required for evoked response testing.
U.S. Pat. No. 5,243,281, assigned to Neuromag Oy, Finland, discloses a superconducting magnetometer or gradiometer for measuring magnetic fields generated by the human brain which are detected simultaneously over the entire skull. A single dewar flask contains an array of detecting coils and SQUIDS. The SQUID gain is increased using positive feedback. Integrated elements are used, each element comprising a SQUID and a magnetometer or pick-up coil. The dewar is shown as having a cylindrical body with a head-shaped (helmet-shaped) cavity at its lower end (dewar tail).
Alternative dewars with a plurality of magnetometers or gradiometers arranged in a helmet-shaped configuration are shown in U.S. Pat. Nos. 5,339,811 and 5,713,354 to Biomagnetic Technologies. The U.S. Pat. No. 5,713,354 uses the term "biomagnetometry" to refer to the measurement of magnetic fields arising from electrical brain waves and "biomagnetometer" to refer to the device, including the SQUID detectors, which is used to measure such magnetic fields. The above-mentioned patents are incorporated by reference.